U.S. Pat. No. 3,842,194 of Clemens discloses a video disc having a playback system utilizing variable capacitance. In one configuration of the Clemens system, information representative of the recorded picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc record. For example, groove widths about 2.6 micrometers and groove depths of about 0.5 micrometer may be used. The disc is coated with a lubricant layer. During playback a pickup stylus about 2.0 micrometers wide having a thin conductive electrode thereon, for example about 0.2 micrometer thick, engages the groove of the record as it is rotated by a supportive turntable. Capacitive variations between the stylus electrode and the record surface are sensed to recover the pre-recorded information.
Keizer, in U.S. Pat. No. 4,162,510, discloses a novel keel tipped pickup stylus structure. The keel tipped pickup stylus comprises a dielectric support element having a body, a constricted terminal portion and shoulders interconnecting the body with the constricted terminal portion. The electrode surface is remote from the prow end of the stylus.
The styli of Clemens and Keizer employ an electrode substantially perpendicular to the record disc surface. Problems have been encountered with these styli caused by the interference of low frequency signals associated with the audio signal with the video signal. This low frequency interference, called sound-beat interference, causes the picture quality to degrade and the picture to break up.
One method of overcoming this problem is disclosed in Clemens et al., U.S. Pat. No. 3,930,117. In the Clemens et al. patent, the metal electrode layer is overcoated with a dielectric layer. The overcoated dielectric layer serves to balance the fringing capacitance dielectric paths of the dielectric support element. It is believed that the imbalance between the path through the dielectric support element and the path through the air contributes to sound-beat interference.
In the copending application of Mindel, Video Disc Playback Stylus, Ser. No. 68,505, filed Aug. 22, 1979, abandoned on or about Mar. 20, 1981, the sound-beat problem is also addressed. Mindel oxidizes the outer portion of the metal conductive electrode layer. The oxidized metal layer serves to balance the dielectric paths.
The Clemens et al. and Mindel styli require additional deposition or chemical steps. It would therefore be advantageous to overcome the sound-beat interference problem without requiring additional processing of the stylus.